1. Field of the Invention
The present invention relates to a neutral beam etching device, and more particularly, to a neutral beam etching device which separates and accelerates a plasma using a grid.
2. Description of the Related Art
A neutral beam etching device uses a plasma state material to etch a wafer. In order to obtain energy sufficient to etch the wafer, the neutral beam etching device has to accelerate a plasma generated in a predetermined area thereof.
A method for accelerating a plasma has been developed for the purpose of studying an ion engine of a rocket for space travel and nuclear fusion, and has been recently used to etch a wafer in the process of manufacturing a semiconductor.
The plasma is a gas that is separated into an electron having negative charges and an ion having positive charges at high temperature. The plasma has high efficiency of charge separation and also is electrically neutral because the positive charge equals to the negative charges. Generally, the three states of matter include solids, liquids, and gases. The plasma is known as the fourth state of matter. Most types of matter are converted from the solid state to the liquid state and from the liquid state to the gaseous state as temperature increases. The gases are separated into electrons and atomic nuclei at tens of thousands of degrees Celsius (° C.) and thus enter a plasma state. Since the plasma state is neutral, an etching device using the plasma is called a neutral beam etching device.
FIG. 1 is a view illustrating a conventional neutral beam etching device. Referring to FIG. 1, the neutral beam etching device adopts a phase matching method and so it is called a ‘traveling wave engine’. Referring to FIG. 1, the neutral beam etching device comprises an external coil 10, an inner coil 20, a discharge coil 30, an external cylinder 40, an inner cylinder 50, a cover 60 and a channel 70.
The cover 60 bridges between the external cylinder 40 and the inner cylinder 50 such that the channel 70 is formed in a space between the external cylinder 40 and the inner cylinder 50. The discharge coil 30 is formed on a top of the cover 60.
The external coil 10 and the inner coil 20 comprise a plurality of coils. The coils coaxially wind around the channel 70 in parallel to one another. In FIG. 1, each of the external coil 10 and the inner coil 20 comprises three coils 1, 2, 3.
When electric currents are applied to the external coil 10, the inner coil 20 and the discharge coil 30, a magnetic filed is generated inside the channel 70. The generated magnetic field generates an induced current according to Maxwell's equations and the generated induced current converts the gases existing in the channel 70 into the plasma state. Accordingly, a plasma is generated in an upper portion of the channel 70.
The plasma generated in the upper portion of the channel 70 is accelerated in the direction of outlet (marked by the arrow) by applying currents to the external and inner coils 10 and 20 winding from the upper portion of the channel 70 to the outlets in sequence. That is, the currents are applied in the order of a first coil 1, a second coil 2 and a third coil 3. By applying the currents in order to the first, second, and third coils 1, 2, 3, a slope of magnetic field is generated at the channel 70 such that the plasma is accelerated.
However, the conventional neutral beam etching device cannot sufficiently accelerate the plasma generated in the upper portion of the channel 70 and thus does not have an energy sufficient to etch a wafer. Accordingly, a satisfactory etching performance cannot be achieved.
Also, phases of currents to be applied to the external and inner coils 10 and 20 have to be adjusted and matched so that the slopes of magnetic fields are generated in the channel 70 in sequence. However, if the plasma generated in the upper portion of the channel 70 has a low initial velocity, a difference between phases becomes larger and thus it is difficult to adopt the phase matching. If a small frequency of driving current is set for the initial velocity, the discharge coil 30 does not smoothly transmit energy to the plasma and thus plasma generation efficiency deteriorates. As a result, an etching efficiency deteriorates.